THE BLACK SPOT OF ROSES 
The black spot is a disease peculiar to the rose, and appears wherever 
this ornamental is grown, in temperate and tropical regions, in greenhouse 
and garden. It is, next to the powdery mildew, the most serious disease 
of roses. Few varieties appear to be immune. 
SYMPTOMS 
The lesions of this disease are confined to the leaves. Examine the 
material provided. OBSERVE :— 
1. That some of the affected leaflets show isolated, more or less 
circular black spots. : 
2. That the margins of these spots are radiate-fibrillose. Study 
with the hand-lens. 
3. The distribution of the lesions. Do they occur on both sides 
of the leaflet? Are they marginal or located more toward the center of the 
blade? 
DRAW a leaf showing the character of the isolated black spots. 
4, That some leaflets are nearly or quite covered by the lesions. 
Explain how this comes about. 
5. That the lesions vary somewhat on different varieties of 
roses. In what respects? Show this in SKETCHES or in NOTES. 
The leaves of some varieties become yellow very soon after the spots 
develop. In others there is little or no yellowing but in either case pre- 
mature defoliation occurs. The early loss of foliage weakens the plants. 
ETIOLOGY 
The cause of this disease is an ascomycetous pathogene, Diplocarpon 
Rosae (Libert) Wolf, the conidial form of which has long been known under 
the name of Actinonema Rosae (Libert) Fries. 
Life-history. The life-activities-of this pathogene are perfectly cor- 
related with the seasonal changes of the temperate zone, although the 
family, Microthyriaceae, to which this fungus belongs, is largely tropical. 
The Primary Cycles are initiated in the spring by inoculum from the 
overwintered infested leaves on the ground. 
Pathogenesis. The inoculum consists of ascospores produced in 
fruit-bodies in the fallen leaves. Remove a bit of tissue containing some 
of these minute fruit-bodies, crush, and under the microscope, OBSERVE :— 
; 6. The two-celled ascospores, some floating free, others still 
within the asci. DRaw or if material is not available copy from Bot. Gaz. 
54, pl. XIII, fig. 16-17. 
The ascospores do not appear to be shot into the air but simply ooze 
out on the surface of the old leaf. How may they reach the unfolding 
leaves of the plant? These ascospores will germinate only on the surface 
of living rose leaves. copy Bot. Gaz. 54, pl. XIII, fig. 18. 
The germtube penetrates the cuticle and gives rise to hyphae which, 
spreading just beneath it over the epidermal cells, form radiating strands 
of subcuticular mycelium. 
Examine one of the spots on the leaves provided and MAKE ouT:-— 
7. These fine branched radiating mycelial strands. What 
color are they? 
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